1. Field of the Invention
The present invention relates to a nickel-metal hydride secondary cell and a negative electrode therefor.
2. Description of the Related Art
Nickel-metal hydride secondary cells have high capacity and superior environmental safety, compared with nickel-cadmium secondary cells, and thus have come to be used in a variety of applications, such as various types of portable devices and hybrid electric vehicles.
A hydrogen absorbing alloy used in the negative electrode of such a nickel-metal hydride secondary cell is capable of storing an amount of hydrogen 1000 times or more the volume of the alloy and is one of important constituent materials of the nickel-metal hydride secondary cell. Hydrogen absorbing alloys generally used include, for example, a LaNi5 hydrogen absorbing alloy, which is a rare earth-Ni hydrogen absorbing alloy with AB5 structure including a CaCu5-type crystal as its main phase, and a hydrogen absorbing alloy with AB2 structure including, as its main phase, a crystal of Laves phase containing Ti, Zr, V and Ni.
Because of a wide variety of applications, there has been a demand for nickel-metal hydride secondary cells having even higher capacity. However, the hydrogen storage capabilities of the existing hydrogen absorbing alloys mentioned above are not necessarily high enough to meet the demand for increased capacity.
In recent years, there has been proposed a rare earth-Mg—Ni hydrogen absorbing alloy with a composition obtained by substituting Mg for part of the rare-earth element in a rare earth-Ni hydrogen absorbing alloy, in order to improve the hydrogen storage capability of the hydrogen absorbing alloy. This rare earth-Mg—Ni hydrogen absorbing alloy is capable of storing a large amount of hydrogen gas, compared with conventional rare earth-Ni hydrogen absorbing alloys (cf. Unexamined Japanese Patent Publication No. 11-323469).
A nickel-metal hydride secondary cell using the above rare earth-Mg—Ni hydrogen absorbing alloy as its negative electrode material has high capacity and is also characterized by suppressed self-discharge and prolonged cycle life, as compared with conventional secondary cells. However, just using the rare earth-Mg—Ni hydrogen absorbing alloy is still not enough to meet the demand for nickel-metal hydride secondary cells having satisfactory self-discharge characteristics and cycle life characteristics.
One cause of self-discharge of the nickel-metal hydride secondary cell is dissociation of hydrogen, for example. Hydrogen dissociated from the hydrogen absorbing alloy of the negative electrode diffuses into the alkaline electrolyte solution, reaches the positive electrode and reduces Ni(OH)2, which is a positive electrode active material, thus causing self-discharge.
On the other hand, one cause of shortening in the cycle life of the nickel-metal hydride secondary cell is ease of cracking of the hydrogen absorbing alloy, for example. Specifically, as hydrogen is repeatedly stored in and released from the hydrogen absorbing alloy of the negative electrode due to charging and discharging of the secondary cell, the hydrogen absorbing alloy cracks and pulverizes. When the hydrogen absorbing alloy cracks, a large number of new surfaces having high reactivity are formed in the alloy. The electrolyte solution in the cell reacts with the newly formed surfaces, so that the hydrogen absorbing alloy is oxidized and thus deteriorated. During the reaction of the electrolyte solution with the newly formed surfaces, the electrolyte solution is consumed and reduced, with the result that the internal resistance of the cell increases, causing lowering of electrical conductivity. Such a phenomenon is generally called dry-out. That is to say, a cell using an easy-to-crack hydrogen absorbing alloy is prone to dry-out and becomes difficult to discharge at a stage where the number of times charging and discharging have been repeated is relatively small, resulting in shortening of the cycle life.
Thus, various researches have hitherto been conducted to solve these problems and to further improve cell characteristics.
However, currently available cells still do not have satisfactory self-discharge characteristics and cycle life characteristics.